It is well known to make a vessel with silica grain in a rotating mold. While the mold rotates, grain is formed therein in the shape of a crucible having a uniform wall thickness. A set of electrodes is then lowered into the mold and high voltage is applied to create a plasma gas cloud around the electrodes, which heats the grain and ultimately fuses them into the crucible.
Such crucibles may be used, e.g., to melt metallic silicon, from which a single crystal ingot may be drawn. These ingots are sliced to provide the silicon wafer on which semiconductor circuits are formed. In other applications, fused silica crucibles are used to melt metallic silicon, which is then poured into a mold to create a polycrystalline silicon ingot. Poured ingots are sliced to make solar cells. The crucible must include a nozzle in applications that require pouring.
Prior art nozzles are formed on crucibles by first making the crucible as described above. After the crucible is formed and cooled, a diamond drill is used to drill a hole through a wall thereof, typically in the center of the bottom of the crucible. A preformed quartz glass nozzle is then welded to the crucible over the hole, thus providing a crucible from which liquid may be discharged via the nozzle.
High heat must be applied to the crucible to weld the nozzle to it. This localized high heat creates stress around the nozzle. If the welded structure is not annealed, the vessel may crack. As the thickness of the vessel increases, the risk of cracking does as well. If the thickness exceeds 3 mm or the crucible diameter exceeds 300 mm, the risk of cracking is very high. Annealing the welded structure takes about 10 hours while fusing the crucible in the rotating mold takes only about an hour.